On a busy summer day as many as 2,500 boats spill from the mouth of Marina del Rey and into the Pacific Ocean.

In order to reach the open sea, boats must turn right to avoid the breakwater that guards the mouth of the harbor. Here, boats under sail and those using engines sometimes cross paths. This can be confusing and dangerous, especially when an inexperienced sailor or bellicose yachtsman becomes involved, boaters said. The scene has been compared to the San Diego Freeway at rush hour.

The traffic jam has worsened this summer as sandy shoals at the mouth of the harbor have reduced the navigable waters by as much as a third. And despite pleas from county officials and boaters, the Army Corps of Engineers has announced that it will not remove the shoals.

"It's very disappointing," said Larry Charness, an administrator with the county's Department of Beaches and Harbors. "It's imperative that some dredging be done on both the north and south jetties to clear a space."

The shoals, sandbanks washed in by the tide and a nearby creek, extend from the end of the twin jetties that form the main harbor channel.

The Corps of Engineers does not have enough money in its budget to dredge the mouth of the harbor, which would cost an estimated $1.8 million, according to Chuck Holt, chief of the corps operations branch that maintains 15 Southern California harbors. Maintenance of recreational harbors such as Marina del Rey receives lower priority than maintaining reservoirs or military and commercial harbors, according to another corps official.

Reagan administration budgets also have provided less money in general for maintaining navigational channels, according to Betty Varner, an analyst with the Corps of Engineers. In the fiscal year beginning this October only California recreational harbors with the most severe shoals will be dredged. Those include the Santa Cruz, Santa Barbara and Channel Islands harbors, Varner said.

The Marina del Rey channel will be dredged in the next year only if the shoals become significantly larger or if money is left over from other projects, the officials said.

In the meantime, boaters have less room to maneuver when they leave and enter the harbor.

"Anything that restricts the amount of passage available creates a certain amount of hazard for us," said Marjory Gandelman, vice commodore of the Del Rey Yacht Club. "We have been very anxious about the fact that it has not been kept dredged and open."

Gandelman said the channel is more crowded than at any time since she began boating in the marina in 1964. "There will still be room to enter and leave safely," she said. "But sometimes we will stand and wait so that there is sufficient room to go in and out."

"It hasn't caused any accidents that I know of personally," said Fred Brown, commodore of the California Yacht Club. "But the problem is that it has always been congested, and (now) when you go out the channel and head north you are confronted with the shoal area. I'm amazed there have not been accidents."

Gandelman said boaters will take "more concerted action" to demand dredging if it is delayed much longer.

County Harbor Patrol boats regularly measure depth of the water at the end of the levees and then extend a row of white and red buoys to mark where the channel is too shallow for boats. "As it continues to narrow, it becomes more and more difficult and it requires more and more skill for the skippers to get through," said Harbor Master Joe Callanan.

Vessels occasionally run aground when skippers ignore the buoys, according to assistant harbor master David Agondo. But Agondo said there has not been an accident attributed directly to the narrowing of the channel.

In addition, a buildup of sand has made the water too shallow for motor boats to use about half of the outbound lane reserved for them along the north jetty, Agondo said.

The entrance to Marina del Rey has been dredged every five to seven years since the harbor opened in 1965.

Dredging was needed in 1981 to clear the channel between the breakwater and the south jetty. The Harbor Patrol closed the channel for more than a year after severe rainstorms washed tons of silt and debris from Ballona Creek to the ocean bottom, making the water too shallow for navigation. The north entrance to the channel remained open.